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1. Field of the Invention
This invention relates generally to a heat exchanger, and more particularly to a heat exchanger apparatus for transferring heat energy between a solid surface and a fluid, which includes liquids and gases.
2. Description of the Related Art
Heat exchangers that transfer heat energy from one fluid, such as a liquid, and another fluid, such as a gas, are well known. Heat exchangers are used commonly to pre-heat or pre-cool fluids in a machine. For example, a well known heat exchanger is an automobile radiator, in which liquid coolant, which has been heated by the combustion of fuel, is pumped into thin-walled passages made of thermally conductive material. Air passes over the outer surfaces of the passages, thereby removing heat during the contact between the air molecules and the outer surfaces of the passages. Thus, heat is exchanged between the liquid in the radiator and the air around the radiator.
In other mechanisms, it is desirable to transfer heat very efficiently between a fluid, such as a gas, and a solid surface. Whether the heat is then transferred to another fluid is irrelevant to the functioning of the mechanism. Within a Stirling cycle engine, for example, a displacer and a piston reciprocate to produce kinetic energy from thermal energy. The process involves the displacement of gas by a displacer within the housing of the engine between a warm end and a cool end. Conventional Stirling cycle engines have axially oriented passages that direct the gas, as the displacer displaces it, through or around the displacer. However, these axial passages do not transfer heat as efficiently as they could. Therefore, the need arises for a more effective heat transfer structure to transfer heat energy between a fluid and a solid surface.
The invention is a heat exchanger apparatus for transferring thermal energy between a fluid and a first wall surface. The apparatus comprises a second wall having a wall surface spaced from and facing the first wall surface to form a gap between the first wall surface and the second wall surface. A first elongated slot is formed in the second wall. The first slot has an opening extending into the gap, and the first slot is in direct fluid communication with a fluid source. A second elongated slot is formed in the second wall spaced laterally from the first elongated slot. The second slot has an opening extending through the second wall surface into the gap. The second slot is in direct fluid communication with a fluid destination.
In a preferred embodiment, the apparatus comprises a second wall having an annular wall surface spaced from and facing the first wall""s annular surface to form an annular gap therebetween. A first elongated slot is formed in the second wall and opens into the gap. The first slot has an axial component of orientation and is in direct fluid communication with a first fluid reservoir. A second elongated slot is formed in the second wall spaced circumferentially from the first elongated slot and opening into the gap. The second elongated slot has an axial component of orientation and is in direct fluid communication with a second fluid reservoir.
In another preferred embodiment, the apparatus comprises a second wall having a wall surface spaced from and facing the first wall surface to form a gap therebetween. A first elongated slot is formed in the second wall. The first slot opens into the gap. A second elongated slot is formed in the second wall spaced laterally from the first elongated slot. The second slot opens into the gap.
A first fluid passageway extends at least partially along the second wall and in direct fluid communication with the first slot. A second fluid passageway extends at least partially along the second wall spaced from the first circumferential passage and in direct fluid communication with the second slot.
Advantageously, the fluid flows over the wall surface, with which thermal energy is transferred, along a short, wide flow path. Causing the fluid to so flow enhances the transfer of thermal energy, because it maintains a large temperature differential between the fluid and the wall surface that is relatively constant over the entire flow path.
The invention is an arrangement of structures that provides significant advantages. It is well known that fluid film heat transfer rate is proportional to the reciprocal of the gap through which the fluid flows. It is therefore preferred to make the gap as small as the maximum permissible pressure drop will allow in order to increase heat transfer per unit area. The pressure drop is proportional to the velocity of flow of the fluid, which is reduced in the invention because the invention provides an arrangement of a plurality of parallel passages through which the fluid flows. Thus, the overall pressure drop per unit of fluid flow is much lower than for a simple axial flow annulus of the same radial gap. The ratio of pressure drops for the same heat transfer is roughly proportional to the reciprocal of number of distributed paths cubed. Therefore, for example, if the number of parallel paths in a device embodying the present invention is four, then the pressure drop is about {fraction (1/16)}th of the pressure drop that would exist in a simple axial flow annulus of the same total area and heat transfer.
This reduction in pressure drop results in smaller gaps and higher heat transfer rate for a given total fluid flow and pressure drop. As a result, the invention results in high heat transfer rates in small gaps without the losses normally resulting from high pressure drops. Additionally, since the fluid is immediately adjacent the wall of the device, for example the pressure wall of a Stirling cycle cryocooler, there is no need for another interface, such as between fins and a wall, and the corresponding disadvantages of brazing at the interface, which causes metallurgical problems.